1. Field of the Invention
Example embodiments of the present invention relate to a nonvolatile memory device, an array of nonvolatile memory devices, and methods of making the same.
2. Description of the Related Art
Recently, studies have been made on semiconductor memory device that increase the number of memory cells per area (e.g., integration density), increase operating speed, and/or that can be driven with lower power.
In general, a semiconductor memory device may include many memory cells that are connected through circuits. In a dynamic random access memory (DRAM), which is a representative semiconductor memory device, a unit memory cell may be comprised of one switch and one capacitor. Such a DRAM may be advantageous due to its high integration density and/or fast operating speed. However, when the power supply is cut off, a DRAM loses all stored data.
By comparison, some nonvolatile memory devices, for example, a flash memory device, can retain stored data even if power supply is interrupted. Although a flash memory device may have a nonvolatile characteristic, it may also have lower integration density and/or slower operating speed than a volatile memory device.
Several nonvolatile memory devices, for example, a magnetic random access memory (MRAM), a ferroelectric random access memory (FRAM), and a phase-change random-access memory (PRAM) are currently under study.
An MRAM stores data using a change of magnetization direction in a tunnel junction and an FRAM stores data using a polarization direction of a ferroelectric material. Although MRAMs and FRAMs have advantages and disadvantages, they both basically have higher integration density, faster operating speed, and/or can be driven with lower power as described above. Research is also being carried out to improve the data retention characteristic of MRAMs and FRAMs.
A PRAM may store data using one or more of a material's characteristic, namely, a variation in resistance with respect to a phase change. A PRAM may include one resistor and one switch (transistor). The resistor used for the PRAM may be a chalcogenide resistor, which may change from a crystalline state to an amorphous (and vice versa) according to a temperature that is controlled when the resistor is formed. A PRAM may be based on the principle that a crystalline resistor is typically more resistive than an amorphous resistor. In fabricating a PRAM using a conventional DRAM process, performing an etch process may become more complicated and/or takes more time. Accordingly, the productivity of such memory devices may decrease and/or the cost of production may increase, thus weakening the competitiveness of the devices.